FIG. 7 shows an example of conventional thermal printhead (See Patent Document 1, for example).
The thermal printhead B is applicable to printing on thermal recording paper and printing on standard recording paper using an ink ribbon. The thermal printhead B performs printing on recording paper by repeating dot-printing operation in a primary scanning direction X (from left to right in FIG. 7) while transferring the recording paper in a secondary scanning direction Y (from below to above in FIG. 7).
Specifically, the thermal printhead B heats thermal recording paper dot by dot to directly perform printing on the recording paper or heats an ink ribbon dot by dot to transfer the ink of the ink ribbon to recording paper to perform printing on the recording paper. The thermal printhead has the function of so-called preheating in the printing process. Preheating is to preliminarily heat thermal recording paper or an ink ribbon by an auxiliary heat-producing resistor section 93 at a temperature which does not cause printing directly before heating the thermal recording paper or the ink ribbon by main heat-producing resistor section 92 for dot printing.
To perform printing on thermal recording paper or printing by heating an ink ribbon, the temperature of the thermal recording paper or the ink ribbon needs to be raised from room temperature to a temperature for printing. Therefore, when printing is to be performed while transferring the recording paper or the ink ribbon, the transfer speed of the thermal recording paper or the ink ribbon is limited to secure time for the temperature rise.
However, by performing preheating directly before the dot printing operation of the thermal recording paper or the ink ribbon, the temperature of the thermal recording paper or the ink ribbon can be raised in advance to a temperature lower than the temperature for printing. Therefore, by the preheating, the time taken for the temperature rise in the dot printing operation can be shortened, which makes it possible to increase the transfer speed of the thermal recording paper or the ink ribbon.
Since an ink ribbon is generally very thin, the rapid temperature rise of the ink ribbon over a wide area in the printing process may result in thermal expansion of the ink ribbon and hence the formation of wrinkles in the ink ribbon. However, by the preheating, the rapid temperature rise of the ink ribbon is prevented, whereby the formation of wrinkles in the ink ribbon is prevented.
The thermal printhead B includes a substrate 91 on which a plurality of main heat-producing resistor sections 92, a plurality of auxiliary heat-producing resistor sections 93 and electrodes 94-96 for energizing these sections are provided. Each of the main heat-producing resistor sections 92 is a heating element for performing the printing of one dot, whereas each of the auxiliary heat-producing resistor sections 93 is a heating element for preliminarily heating thermal recording paper or an ink ribbon in performing printing by heating the thermal recording paper or the ink ribbon by the main heat-producing resistor section 92.
The plurality of main heat-producing resistor sections 92 are arranged in a row in the primary scanning direction X at predetermined intervals, so are the auxiliary heat-producing resistor sections 93. Each of the auxiliary heat-producing resistor sections 93 is positioned upstream from each of the main heat-producing resistor sections 92 in the secondary scanning direction Y (upstream in the direction in which recording paper is transferred in the printing process) and connected in series to the main heat-producing resistor section 92 by the electrode 96.
The distance D4 between the main heat-producing resistor section 92 and the auxiliary heat-producing resistor section 93 is made shorter than the length L4 of the main heat-producing resistor section 92 in the secondary scanning direction Y. This is because, when the distance D4 is shorter than the length L4 of the main heat-producing resistor section 92, the preheating by the auxiliary heat-producing resistor section 93 works effectively even when the recording paper is transferred at high speed in the secondary scanning direction Y.
The width Wa of each of the auxiliary heat-producing resistor sections 93 is larger than the width Wb of each of the main heat-producing resistor sections 92. The main heat-producing resistor sections 92 and the auxiliary heat-producing resistor section 93 which are spaced in the secondary scanning direction Y are connected in series and heated by applying the same current. Therefore, by making the width Wa larger than the width Wb, the resistance of the main heat-producing resistor section 92 becomes higher than that of the auxiliary heat-producing resistor section 93 so that the amount of heat production of the main heat-producing resistor section 92 becomes larger than the amount of heat production of the auxiliary heat-producing resistor section 93.
Specifically, the resistance of a resistor section is directly proportional to the dimension (length) in the direction in which current flows (secondary scanning direction Y in FIG. 7) and inversely proportional to the dimension (width) in the direction (primary scanning direction X in FIG. 7) which is perpendicular to the current flow direction. Therefore, the resistance of the auxiliary heat-producing resistor section 93 is made lower than that of the main heat-producing resistor section 92 by making the width Wa of the auxiliary heat-producing resistor section 93 larger than the width Wb of the main heat-producing resistor section 92.
Patent Document 1: JP-A-H08-150750
In the conventional thermal printhead B, the distance D4 between the main heat-producing resistor section 92 and the auxiliary heat-producing resistor section 93 is small and made shorter than the length L4 of the main heat-producing resistor section 92 in the secondary scanning direction Y, which causes the following problems.
The main heat-producing resistor section 92 and the auxiliary heat-producing resistor section 93 which correspond to a dot to be printed are controlled to be energized and produce heat simultaneously. Therefore, the heat produced at the main heat-producing resistor section 92 is transmitted to the auxiliary heat-producing resistor section 93 through the electrode 96, which disadvantageously raises the temperature of the electrode 96 and influences the temperature of the auxiliary heat-producing resistor section 93.
Particularly, to print a line extending in parallel with the secondary scanning direction Y on a recording sheet, the same main heat-producing resistor section 92 and the relevant auxiliary heat-producing resistor section 93 are continuously energized for continuous heat production. In such a case, the heat production at the main heat-producing resistor section 92 and the auxiliary heat-producing resistor section 93 are continued, so that the heating temperature at the auxiliary heat-producing resistor section 93 becomes higher than the predetermined preheating temperature due to the influence of the heat production at the main heat-producing resistor section 92.
Moreover, even when the energization of the same heat-producing resistor section 92 and the auxiliary heat-producing resistor section 93 is not continuous, the main heat-producing resistor section 92 is not sufficiently cooled when the printing operation is repetitively performed with short intervals. Also in this case, the heating temperature of the main heat-producing resistor section 92 influences the heating temperature of the auxiliary heat-producing resistor section 93 to make the temperature higher than the predetermined preheating temperature.
Therefore, in the printing on thermal recording paper by using the conventional thermal printhead B, the heating temperature of the auxiliary heat-producing resistor section 93 or the temperature of the electrode 96 may exceed the intended temperature. In such a case, the thermal paper is heated by these elements and colored, which results in deterioration of the print quality.
With the conventional thermal printhead B, the above-described problem also occurs in the printing on regular recording paper which is performed by heating an ink ribbon and melting the ink for transferring to the recording paper. Specifically, the temperature of the auxiliary heat-producing section 93 or the electrode 96 rises and exceeds the predetermined temperature so that the ink of the ink ribbon is melt and transferred to the recording paper, which leads to the deterioration of the print quality.
In addition, since the conventional thermal printhead B has the preheating function, the formation of wrinkles in the ink ribbon can be lessened as compared with a thermal printhead which does not have preheating function. However, as noted before, the distance D between the main heat-producing resistor section 92 and the auxiliary heat-producing resistor section 93 is small in the conventional thermal printheadB. Therefore, when the ink ribbon and recording paper are transferred at high speed, the preheating of the ink ribbon by the auxiliary heat-producing resistor section 93 and the heating of the ink ribbon by the main heat-producing resistor section 92 for printing are performed substantially successively, so that the rapid temperature increase occurs in two stages. Therefore, the formation of wrinkles in the ink ribbon cannot be sufficiently prevented by the preheating.